||03-05-2007 03:41 PM
How did you upgrade the kernel? If you used yum or just rpm to install a new kernel .rpm package, chances are you do have the old kernel there too -- check your /boot for what files reside there. Basically because the kernel can be installed from a .rpm binary package, you could just find your preferred version of the kernel from the web, download it (and it's dependencies -- correct versions of them, note) and install them using rpm
I guess that explains which command line options you use to downgrade and possibly force downgrading. Because rpm doesn't like you having two different versions of the same package installed simultaneously, you may have to do some tricks to downgrade (not difficult), but it doesn't prevent you from having multiple kernels around. Generally when upgrading a kernel you should never remove the old kernel before you're 100% sure (or even then) that the new one works and you don't need the old one anymore. That's why, when you upgrade your kernel in a binary distribution like Fedora, it doesn't (read: should not) usually remove the old kernel files.
If you happen to have the old kernel files in your /boot you can simply use your bootloader (GRUB nowadays I guess) to boot one of the older ones. And if you like that permanent, configure your bootloader to do so. There are sites on the web with more precise information about this (kernel.org too), and what it means when you upgrade a kernel or have multiple kernels which you want to use, so go ahead -- search and read. My advice (though this should have been heard before, and not after, this :) like usually) is that if you're doing a kernel upgrade -- especially on a server, and especially especially on an "important" "work" "server", first test the new kernel on an identical test environment for a long enough time (how long is that?;) ) before doing the upgrade on the work machine, and when you do, always have the old kernel there for backup.